1. Field of Use
This invention relates to an improved apparatus for heating and curing powder coatings on porous wood products, such as medium density fiberboard (MDF). More specifically, the invention relates an improved catalytically powered oven employing a novel arrangement of infrared catalytic heaters for heating and curing powdered coatings on MDF board.
2. Description of Prior Art
For the past twenty-five years, the powder coating of metal parts has become a popular method of finishing. There are numerous suppliers of powder coating catering to all segments of the metal industry; ranging from automotive to architectural to marine applications. A typical method of applying powder to metal parts is to charge the powder particles with a charge via a spray gun. These charged particles are then attracted to metal parts that are earthed via a grounded hanging device on a conveying system.
Wood, or engineered wood products (EWP), such as medium density fiberboard (MDF) are not naturally as conductive as typical metal parts. MDF is made conductive by preheating, for up to 3 minutes, the MDF to a range that is between about 150 and 250 degrees Fahrenheit. Preheating the MDF activates the moisture content of the MDF (typically about 5-10%) causing it to become conductive. Thus, charged powder will attach to a properly grounded MDF board.
Once the powder is attached to the MDF board, the method of curing has been by either heating the powder in a convection oven for a certain period of time or by infrared heat for a period of time that is less than that of a convection oven. The infrared heat source has been either electric resistance heaters or catalytic heaters. In recent years, catalytic heaters have attracted considerable attention as the preferred choice of infrared heat sources.
MDF board is available in various thicknesses ranging from one-quarter (¼) inch through to two inches, for example. With all thicknesses, the face surfaces of the MDF board are of a considerable higher density than the core of the board. The greater the thickness of the MDF board, the greater the difference is between the core density and the face surface density. MDF board has a certain amount of naturally occurring porosity within the board structure and hence a characteristic moisture content. The greater the thickness, the greater the porosity due to the lower core density.
Curing powder coatings on medium density fiberboard (MDF) using an infrared heat source has given rise to certain difficult problems. When heating a piece of powder coated MDF board to cause the powder to cure, the board is typically hanging in a vertical position. As the board heats up, the entrapped moisture expands and out-gases through the edges of the board, typically from the center of the core in the area of lowest density. During the curing process using a conventional catalytic heating oven, the face surfaces of the board are easily heated, while the edges, especially the vertical edges, do not receive a full direct line of site of infrared energy. As a result, the edges of the board are the last to cure as compared to the face surfaces. This leads to an occurrence where the expanding moisture, which is out-gassing from inside the board, bubbles and forms blisters along the side edges of the board. These blisters occur because the powder at the edges has not reached a degree of cure, as compared to the face of the board, which would prevent the blisters from forming.
Furthermore, powder coatings, going through the curing process, first turn to liquid and then a gel stage followed by a curing stage where the powder reaches its full cured properties. However, the liquefied powder will be drawn into the edges of the MDF in a similar manner to edge grain on wood absorbing liquids. Consequently, wood fibers appear and present an undesirable different look and feel to that of the coated and cured face sides of the MDF board and EWP's.
Depending on the method of cutting and sanding of the edges of the MDF board the wood fibers will protrude in varying degrees. The degree of this protrusion is dependent on the density across the board thickness and a umber of other factors to do with the physical properties of the board—fiber type and length, percentage and type of glue used, and the MDF board and the EWP's manufacturing process in general.
Thus, the manufacturing and pre-finishing processes for the MDF board, along with the precise application of the powder thickness on the edges, all contribute too many variables that may produce sub-standard edge finishes, resulting in waste and low yields.
To compensate for the issues associated with powder coating the edges of MDF boards the present state of the art employs both a single coat application and a two coat application. In both applications it is the vertical edges that are required to receive a predominate level of infrared heat to allow the powder to flow, seal and cure the edges ahead of the face sides of the board. Generally, a powder prime coat is applied to the edges and faces of the MDF, partially cured, followed by a powder top coat and then the two coats are co-cured together. The end result provides an acceptable edge finish that mitigates, but doesn't eliminate the undesirable variables mentioned above.
Thus, there exists a need for a system and method for the edge treatment of MDF boards and EWPs to maintain a high quality powder coated MDF board while reducing associated manufacturing expenses.